"People in France have a phrase: "staircase wit." In French: esprit de l'escalier. It means that moment when you find the answer, but it's too late. Say you're at a party and someone insults you. You have to say something. So under pressure, with everybody watching, you say something lame. But the moment you leave the party...As you start down the stairway, then-magic. You come up with the perfect thing you should've said. The perfect crippling put-down. That’s the spirit of the stairway."

( Checked Quidditch Through The Ages out of the Hogwarts library  )